As is well known, polyacetal resins have conventionally been used in a number of fields as engineering plastics due to their excellent physical properties (such as desirable mechanical and electrical properties) and their excellent chemical properties (such as desirable chemical and heat resistance characteristics). However, polyacetal resins are required to have further property enhancements as new applications in which polyacetal resins may be used are identified. For example, further improvements are required in the weather resistance properties of polyacetal resins. In this regard, the interior and exterior parts of automobiles and parts for electric appliances tend to experience discoloration, loss of surface smoothness properties (resulting in decreased "gloss"), surface crack formation, and the like when subjected to the deleterious influence of their ambient conditions of use, such as solar rays, fluorescent lamps, rain and air.
It has been proposed to improve the weather resistance of polyacetal resins by incorporating various weather stabilizers in order to overcome the affects mentioned above. However, those instances in which a weather stabilizer is used are not always satisfactory. Although some beneficial weather-resistance can be obtained by adding a weather stabilizer and an acrylic resin to polyacetal base resins, the surface conditions (cracks, gloss, etc.) and the mechanical properties of moldings formed of such compositions are not always satisfactory after exposure to solar rays (ultraviolet rays), rain, and other "outdoor" conditions for prolonged time periods.
Increasing the amount of the weather stabilizer, acrylic resin or the like only results in minimal improvements in the weather-resistance properties of polyacetal resin compositions. In addition, an increase in the amount of such additives usually impairs the composition's otherwise desirable mechanical properties (e.g., tensile strength and Izod impact strength properties). Furthermore, UV-light stabilizers typically impart poor mold release properties to the composition and/or form deposits on the mold surface during molding, thereby reducing the dimensional accuracy and/or appearance of the moldings. As a result, an increase in the time needed to clean the mold increases thereby decreasing the efficiency of the molding operation. Further improvements in the weather stability characteristics of polyacetal reins are therefore still required.
Broadly, the present invention relates to moldable polyacetal molding compositions, and to molded articles formed thereof which include a polyacetal base resin and a weather stabilizing effective amount of a stabilization package comprised of (i) a weather (UV-light) stabilizer for polyacetal resin, (ii) an acrylic resin, and (iii) an oxyalkylene polymer comprised of oxyalkylene units having 2 to 8 carbon atoms.
A preferred polyacetal composition according to the present invention comprises (A) between 45 to 98.89 wt. % of a polyacetal base resin, (B) between 0.01 to 5 wt. % of a weather stabilizer, (C) between 1 to 40 wt. % of an acrylic resin and (D) between 0.1 to 10 wt. % (preferably 1 to 10% by weight) of an oxyalkylene polymer having 2 to 8 carbon atoms per oxyalkylene unit in the polymer chain.